HDU 6434 Problem I. Count

#include<bits/stdc++.h>
using namespace std;
#define LL long long

const int N = 2e7;
const int maxn = N+10;

int T,n;
int primes,prime[maxn],phi[maxn];
bool vis[maxn];
LL ans[maxn];

void init()
{
  int i,j;
  phi[1]=1;
  for (i=2;i<=N;i++)
  {
    if (!vis[i])
    {
      prime[++primes]=i;
      phi[i]=i-1;
    }
    for (j=1;j<=primes&&i*prime[j]<=N;j++)
    {
      vis[i*prime[j]]=1;
      if (i%prime[j])
        phi[i*prime[j]]=phi[i]*(prime[j]-1);
      else
      {
        phi[i*prime[j]]=phi[i]*prime[j];
        break;
      }
    }
  }
  phi[1] = 0;
  ans[0] = 0;
  for (int i = 1; i <= N; i++) {
        if (i & 1) {
            ans[i] = ans[i-1] + phi[i]/2;
        }
        else {
            ans[i] = ans[i-1] + phi[i];
        }
  }
}

int main()
{
  init();
  scanf("%d",&T);
  while (T--)
    scanf("%d",&n),printf("%lld\n", ans[n]);
}